One prior art frequency synthesizer arrangement comprises a digital generator circuit for producing a first series of digital samples which are representative of the signal to be synthesized and are formed by binary words of N bits, and a digital-to-analog converter circuit for producing said signal in analog form.
The present invention also relates to a transmitter station and a receiver station comprising such an arrangement.
An arrangement of this type finds highly significant applications especially for processing sine-shaped signals having a given frequency and for constituting thus a direct digital frequency synthesizer. A description of such an arrangement will be found in U.S. Pat. No. 5,014,231.
A first requirement often imposed on this type of arrangement is that the produced signal must be very pure and a second requirement is that the signal must have no spurious sidebands. These sidebands are basically caused by the sine-shaped and periodic form to be synthesized. To satisfy this first requirement, it is possible to provide a large number of bits for the samples of the first series. For example, to obtain a sine curve having a purity of 84 dB, it is necessary that the output samples of the form generator circuit have 14 bits. This results in the formula: EQU N.sub.b .gtoreq.R/6
where N.sub.b is the number of bits and
R is the ratio in decibels of the level of the sine curve to the levels of the spurious sidebands.
Then, when this type of synthesizer arrangement is intended to be used for high frequencies, for example, of the order of several hundred MHz, the problem is there to find a cost-effective digital-to-analog converter circuit. To satisfy the second requirement, said patent proposes to add a random number to the samples of the first series. If the level of these sidebands is reduced in this manner, it is at the cost of a general degradation of the purity of the signal, and then one cannot but include a costly digital-to-analog converter.